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Phonons by definition require a material to propagate—lattice vibrations can’t happen without a lattice. Yet for a decade theorists have suspected that a phonon could move across a vacuum gap between two materials with the help of a quantum effect known as the Casimir force. Now Xiang Zhang of the University of California, Berkeley, and his colleagues have observed for the first time a phonon moving between two materials spaced hundreds of nanometers apart. Because phonons are energy carriers, the result reveals a new nonradiative route for heat transfer in vacuum.

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